To efficiently operate an engine, the engine needs to be warmed up to an appropriate operation temperature after the start of the engine. Conventionally, a method of stopping circulation of a coolant for cooling the engine to raise the temperature of the engine is used as a warm-up method of the engine. Meanwhile, a method of exchanging heat between the exhaust gas of the engine and the coolant and utilizing the exhaust heat of the engine to warm the coolant to quickly warm up the engine is also used (for example, see Japanese Patent No. 4826502).
An engine cooling system is also proposed, in which a spring relief valve is provided in a connection channel that connects an engine downstream of a first coolant channel passing through an engine and a second coolant channel bypassing the engine and circulating through an exhaust heat collection device and a heater core, a flow rate of a coolant pump is reduced by closing the relief valve to stop distributing the coolant to the engine to warm up the engine, and the flow rate of the coolant pump is increased by opening the relief valve when the temperature of the coolant in the engine may boil during the warm-up or when heating of the vehicle interior is in high demand, so that the coolant passes through the engine from the first coolant channel and flows into the heater core of the second coolant channel, thereby preventing overheating of the coolant in the engine and performing control to satisfy a heating request in the vehicle interior (for example, see Japanese Patent Laid-open Publication No. 2011-99400).
Furthermore, Japanese Patent Laid-Open Publication No. 2014-1654 proposes applying an electromagnetic valve, in which the opening is reduced by applying a voltage, and the opening is increased by cutting off the voltage, in place of the spring relief valve used in the engine cooling system described in Japanese Patent Laid-Open Publication No. 2011-99400. In this system, the valve is opened according to an open request of the electromagnetic valve based on the temperature or the like of the engine regardless of the flow rate of the coolant pump. The coolant passes through the engine from the first coolant channel and flows into the heater core of the second coolant channel. In this way, overheating of the coolant in the engine can be prevented, and control can be performed to satisfy a heating request in the vehicle interior.
When the engine cooling system described in Japanese Patent Laid-Open Publication No. 2014-1654 is used, the fuel efficiency may be improved by closing the electromagnetic valve provided in the connection channel after opening the electromagnetic valve, depending on the operation status of the engine. For example, when the coolant temperature drops to some extent after intermittent stopping of the engine, the fuel efficiency can be improved by closing the opened electromagnetic valve to stop distributing the coolant into the engine to prevent the temperature of the engine from dropping too much. When high load operation is performed just after the start of the engine in a low-temperature state, the electromagnetic valve is opened even before the engine warm-up in order to prevent a local rise in the coolant temperature in the engine. Subsequently, when the load of the operation becomes low before the end of the warm-up of the engine, the engine can be more quickly warmed up by closing the opened electromagnetic valve, and the fuel efficiency can be improved. In this way, there may be a request for closing the opened electromagnetic valve depending on the operation status of the engine in order to improve the fuel efficiency in the engine cooling system described in Japanese Patent Laid-open Publication No. 2014-1654.
However, in the electromagnetic valve described in Japanese Patent Laid-open Publication No. 2014-1654, the coolant flows in a direction such that a valve body separates from a valve seat. Therefore, the valve cannot be closed by applying a voltage to the electromagnetic coil when the coolant is flowing. To close the valve, the coolant pump needs to be temporarily terminated to stop the flow of the coolant, and the voltage needs to be applied to the electromagnetic coil after the valve body is seated on the valve seat. Therefore, when the electromagnetic valve is closed after the intermittent stop of the engine for example, the coolant pump is temporarily stopped to close the electromagnetic valve in the system described in Japanese Patent Laid-Open Publication No. 2014-1654. The function of a heater core installed in a coolant circulation circuit is degraded, and the temperature of the air blown out from a blower of a heating apparatus in the vehicle temporarily drops. This may give the driver a feeling of discomfort. Furthermore, when the electromagnetic valve is closed based on a valve closing request of the electromagnetic valve for restarting the engine warm-up after the load changes from high to low just after the cold start, for example, the coolant pump is stopped, due to closing of the electromagnetic valve, and the coolant in the exhaust heat collection device may boil.
It is an advantage of the present invention to prevent giving a driver a feeling of discomfort and to prevent boiling of a coolant in an exhaust heat collection device in an engine cooling system.